OF VITAL PHENOMENA 95 



he called lipoids. These are cholesterin, lecithin and some other 

 substances that may be extracted from cells with alcohol, and 

 are soluble in ether. Cholesterin is a polyhydric alcohol, crystal- 

 lizing in waxy plates. Lecithin is a glyceride in which one hydro- 

 gen is substituted by phosphoric acid and the other two by fatty 

 acids, one hydrogen of the phosphoric acid being substituted by 

 choline. Lecithine is a waxy substance which swells in water 

 and alters its power of dissolving other substances, finally pass- 

 ing into a colloidal solution. In order to obtain the lipoids in 

 a dry, fluid condition, Overton and others dissolved them in 

 benzol, xylol, toluol, chloroform or oil of turpentine. Many 

 substances which dissolve in one of these mixtures easily also 

 penetrate cells easily, but exceptions are found. The partition 

 coefficients were usually not determined, and a reliable quantita- 

 tive method of estimating permeability was not always used. 



The composition of the plasma membrane remains a mystery. 

 It seems logical to assume that its building stones are selected 

 from the chief constituents of cells, proteins, fats, lecithin, cho- 

 lesterin, and carbohydrates. It is 'a very unstable structure as 

 will be shown later. 



Some attempts have been made to find or construct membranes 

 with the same properties as the plasma membrane. Pascucci 

 (1907) impregnated silk with lecithin and cholesterin and found 

 this membrane to show some of the properties of the plasma 

 membrane of the erythrocytes. It was shown by A. Brown 

 (1909) that the covering of the barley grain has the same perme- 

 ability as some cells, but he has not proved that this property is 

 due to the cellulose it contains or to any known substance. Beut- 

 ner claims to have made membranes with some of the properties 

 of the plasma membrane. These will be described in a later 

 chapter. Traube's membranes are easily permeable to acids and 

 bases; halides and nitrates of alkalis, many amines, urea and 

 H 2 2 . They are poorly permeable to sulphates, phosphates, 

 carbonates and all salts of alkaline earths and heavy metals, salts 

 of many organic acids, four substituted derivatives of NH 3 , gly- 

 cerine, basic and many acid dyes, but impermeable to sugars. 



Overton (1899) studied the permeability of plant cells by 

 means of the plasmolytic method, and arranged substances in 

 groups according to their penetration power as follows: 



